EEG and Epilepsy Monitoring
Core concepts
Know the normal adult waking background (posterior alpha 8–13 Hz, reactive to eye opening), the stages of drowsiness and sleep (vertex waves, sleep spindles, K-complexes), and the activation procedures (hyperventilation, photic stimulation). Distinguish epileptiform discharges (spikes, sharp waves, spike-and-wave) from benign variants such as wicket spikes, 14-and-6 positive bursts, and rhythmic midtemporal theta of drowsiness. Understand the 10–20 electrode system, polarity convention (negative up), and the meaning of referential versus bipolar montages, including phase reversal as a localizing sign in bipolar chains. For monitoring, know the classification of seizures by onset (focal, generalized, unknown), the value of video-EEG correlation, and the principle that ictal patterns evolve in frequency and amplitude.
Traps
Questions often exploit polarity confusion: a focal negativity at a single electrode produces a phase reversal (pointing toward each other) in a bipolar chain that includes it. Another trap is labeling a benign variant as epileptiform. Remember that hyperventilation provokes absence seizures and also normal slowing; photic stimulation drives a photoparoxysmal response only when it is time-locked to the flash. Do not confuse periodic lateralized epileptiform discharges (which are not themselves seizures) with ictal rhythms.
Worked example / rule of thumb
A focal spike at F7 in a longitudinal bipolar chain (Fp1-F7, F7-T3) shows a downward deflection in Fp1-F7 and an upward deflection in F7-T3 - the two channels point toward each other at F7, localizing the negativity there. Rule of thumb: in bipolar montages, the phase-reversing electrode is the source.
Key points
- Negative is plotted up; phase reversal localizes in bipolar chains.
- Separate epileptiform discharges from benign variants before answering.
- Alpha rhythm attenuates with eye opening; absence of reactivity is abnormal.
- Hyperventilation and photic stimulation are standard activation methods.
- Ictal patterns evolve; interictal discharges do not constitute a seizure.
- Know 10–20 electrode placements and referential versus bipolar logic.